no option to add affinity group to virtual network azure

This option have intentionally been replaced by the Region/Location as you said.
Regional Virtual Networks are supposed to be more appropriate than affinity groups.

Obviously, if you still want to bring back the Affinity group, then try this:

Create a Virtual Network from the Management Portal Download the Virtual Network Configuration File

Edit the appropriate file section to get

VirtualNetworkSitename="VNetDemo" AffinityGroup="VNetDemoAG"

instead of

VirtualNetworkSitename="VNetUSWest" Location="West US"

Then, delete the previously created virtual network and import your file in the management portal to recreate the network with your defined settings